1. Field of the Invention
The present invention relates to an organic thin film transistor. In particular, the present invention relates to the organic thin film transistor which is protected electromagnetically, is stabilized characteristics, and is suitable for integration.
2. Description of the Related Art
In a circuit element using an organic semiconductor, the circuit element, which is maintained the characteristics of the organic semiconductor stabilized for a long period of time, and whose endurance was high also for various stress, shocks, etc. from the outside, and which is excellent in reliability, is disclosed (for example, refer to Patent Document 1). The circuit element related to the Patent Document 1 is a circuit element which forms a circuit section including the organic semiconductor on a substrate, and has a sealing can which surrounds the concerned circuit section having predetermined space.
On the other hand, a field effect transistor, which has the structure which can control that characteristics change or deteriorate originated in existence of water vapor of the air, is disclosed (for example, refer to Patent Documents 2). The field effect transistor disclosed in the Patent Document 2 includes: a gate electrode formed on a base substance; a gate insulating film formed on the gate electrode; source/drain electrodes formed on the gate insulating film; and a channel forming region composed of an organic semiconductor material layer formed on the gate insulating film between the source/drain electrodes. A protective layer is formed at least on the channel forming region, and the protective layer has at least a layered structure of a layer which has hygroscopy and a layer which has moisture resistance.
As shown in FIG. 1, the structure of the conventional organic transistor includes: a gate electrode 12 placed on a substrate 10; a gate insulating film 14 placed on the gate electrode 12; an organic semiconductor layer 20 placed on the gate insulating film 14; and a source electrode 16 and a drain electrode 18 placed between the gate insulating film 14 and the organic semiconductor layer 20. In fabricating of the conventional organic transistor, as shown in FIG. 1, the gate electrode 12 is provided in the lower part, next the gate insulating film 14 is formed on the gate electrode 12, and the organic semiconductor layer 20 are formed on the gate insulating film 14. In structure of the conventional organic transistor shown in FIG. 1, it is in the state where the organic semiconductor layer 20 is exposed for the atmosphere.
As for the organic semiconductor material, since change of mobility, degradation of other various characteristics, etc. occur by oxygen and moisture in the air, etc., it is general to intercept from the atmosphere by forming a protective film on the upper part of the organic semiconductor layer 20 (for example, refer to Patent Document 2), or by using the sealing can composed of glass or metal (for example, refer to patent documents 1), as mentioned above.
Moreover, a method of providing a layered structure of an organic insulating layer and a metal layer on the upper part of the above-mentioned organic semiconductor layer 20, and using as a sealing film of the organic transistor is known (for example, refer to the Non-Patent Document 1).
Furthermore, a structure of providing an insulating layer and a metal layer on the upper part of the above-mentioned organic semiconductor layer 20, and using the metal layer as a second gate electrode is known (for example, refer to Non-Patent Document 2).
Patent document 1:                Japanese Patent Application Laying-Open Publication No. 2005-277065 (Pages 3 to 7, and FIG. 2 to FIG. 3)        
Patent document 2:                Japanese Patent Application Laying-Open Publication No. 2005-191077 (Pages 10 to 11, and FIG. 3)        
Non-Patent Document 1:                Technical Research Report of the Institute of Electronics, Information and Communication Engineers, OME2006-120, Vol. 106, No. 439, pp. 65-68        
Non-Patent Document 2:                Technical Research Report of the Institute of Electronics, Information and Communication Engineers, OME2006-56, Vol. 106, No. 183, pp. 33-35        
However, in the methods of the above-mentioned Patent Documents 1 and 2, there was a problem in a viewpoint of electromagnetic protection. That is, when organic transistor technology progresses and integration is performed, a carrier is induced by the electric field and magnetic field which are generated from the approaching element, and then malfunction of the transistor may occur.
Although the electromagnetic protection from the outside is possible in the metal sealing cans, the electromagnetic protection from an element inside of the device is impossible.
In the Non-Patent Document 1, not only the protection from oxygen and moisture in the atmosphere but also the electromagnetic protection from the outside is achieved. However, since the metal layer of the first layer opposes with the source electrode and the drain electrode on both sides of the insulating layer and the organic semiconductor layer, the parasitic capacitance exists. Since the metal layer is separated off electrically not giving potential directly, potential of the metal layer is unstably varied by an electrical change of not only the source electrode but the drain electrode. Since the metal layer adjoining through the insulating layer can act as a gate electrode, if the potential of the metal layer is unstable, it becomes a factor that the current which is not aimed generates at the time where the transistor turns OFF. That is, increase of leakage current, reduction of an ON/OFF ratio, etc. are caused.
In the Non-Patent Document 2, since the metal layer is used giving potential directly taking the electric connection as the second gate electrode, it is not the above-mentioned limitation. However, wiring and electric contact must be independently provided for the second gate electrode, in this case. Although it is expected easily that it is necessary to connect the second gate electrode with a lower layer gate electrode or other electrodes depending on a circuit to be formed, a region for electric contact formation must be ensured in that case, and integration becomes difficult.